cw Measurement of the Optical Nonlinearity of Ammonium Dihydrogen Phosphate

We have measured the value of the element d₃₆ of the nonlinear dielectric tensor of ammonium dihydrogen phosphate for the doubling of the 6328-{} line of the He-Ne laser. Two measurements were made. The first used a collimated single-transverse-mode laser beam (focalspotradius≈0.2 cm) containing several longitudinal modes. The second used a weakly focused single-mode and single-frequency laser (focalspotradius≈0.03 cm). In both experiments the crystal was slowly rotated through the index-matching angle. The value of d₃₆ obtained by either method is 1.36×{}10^-9(±{}12%) in cgs esu. The experiments also verify the factor (2n-1)n for the enhancement of second-harmonic generation when n independent longitudinal modes are present.